Is it possible to be effectful with F[_]: Async instead of IO ?

[ronanM]

No description provided.

[luksow]

Hi, thanks for a very good question that should be probably addressed in README :)

So sadly, at this point - no. IO is bolted hard in the internals. I've tried going tagless final style at the beginning but I had various problems with type interference. Especially directives were awkward to compose when they were typed with F[_]. At some point my friend suggested that this could be possible if wrapped into a trait like

trait DSL[F[_]: Async] {
// everything goes here
}

to avoid that but then, it's quite cumbersome to structure it properly.

I'm of course open to more ideas and even better - contributions :)

[MateuszKubuszok]

Considering that most of the things within the DSL would not be able to use F in a meaningful way (it would use Async or even more powerful type class so not much of a constraint anyway), one can use LiftIO[F] to take the output of DSL and compose it into some Program[F]. And withing the DSL one can also take the resulting F[A] and convert it to IO[A] with some F ~> IO.

Actually, now that I think about it, it can be done with something similar to Dispatcher, which would convert F to IO rather than Future.

Maybe something like:

trait IODispatcher[F[_]]:
  def runToIO[A](fa: F[A]): IO[A]
object IODispatcher:
  def apply[F[_]: Async]: Resource[F, IODispatcher[F]] = Dispatcher[F].map { dispatcher =>
    new IODispatcher[F] {
      def runToIO[A](fa: F[A]): IO[A] = IO.defer {
        val (future, cancel) = dispatcher.unsafeToFutureCancelable(fa)
        IO.fromFuture(IO.pure(future)).onCancel(IO(cancel()))
      }
    }
  }


IODispatcher[F].use { ioDispatcher =>
  // Create Route using DSL.
  // When you have:
  //   fa: F[A]
  // and want to use it in onComplete do:
  //   onComplete(ioDispatcher.runToIO(fa))
  def routes(ws: WebSocketBuilder2[IO]): Route = ...
    
  LiftIO[F].liftIO {
    // Creates and runs server as IO[Unit],
    // which LiftIO will turn to F[Unit]
    EmberServerBuilder
      .default[IO]
      .withHost(ipv4"0.0.0.0")
      .withPort(port"8080")
      .withHttpWebSocketApp(ws => routes(ws).toHttpRoutes.orNotFound)
      .build
      .use(_ => IO.never)
  }
}